The present invention relates to a refractory or fireproof brick useful as a tin bath bottom brick in systems for manufacturing plate glass according to the float method.
In systems for manufacturing plate glass according to the float method, large-format refractory bricks having dimensions of 300.times.600.times.900 mm, for example, are used to receive the tin bath. The bricks form a flat bottom, on which the molten tin rests in a flat layer. The bricks used to date that are based on alumina-silica fireclay are subjected, during service at temperatures ranging from 800.degree. to 1000.degree. C., to corrosion due to the spalling of thin, dish-shaped layers and due to the crack formation and spalling at greater depths parallel to the hot brick surface. The dish-shaped spalling is produced by alkali oxides such as Na.sub.2 O, which emerge from the glass melt and migrate through the molten tin to the refractory material, thus resulting in an increase in volume in the formation of feldspars and feldspar derivatives in the fireclay brick.
The fireclay bricks used as tin bath bottom bricks are supposed to fulfill requirements with respect to strength, and in particular low permeability to gas and high alkali resistance. According to DE 40 13 294 A1, improved fireclay bricks are proposed using a silicate material with a high alkali content.
Refractory basic materials on the basis of sintered magnesia, fused magnesite, forsterite (2MgO.multidot.SiO.sub.2), or magnesiospinel (MgO.multidot.Al.sub.2 O.sub.3) have a high alkali resistance. However, use as a tin bath bottom brick would result in bricks whose high thermal expansion and thermal conductivity are regarded as shortcomings.